This invention relates to character recognition systems. Such systems sense symbols printed in a predetermined type of font and analyze electrical waveforms generated from the sensed symbols. In this analysis certain characteristics of the generated electrical waveforms are compared with prescribed characteristics of predetermined waveform patterns uniquely associated with individual characters in the type font. A characteristic of the electrical waveform generated for comparison with a predetermined ideal waveform may be spacially separated voltage peaks.
1. Field of the Invention
In the art of magnetic ink character recognition, characters or symbols which are to be recognized as unique characters of a given type font are printed on a document with magnetizable ink. This document, upon which the symbols are printed, is transported past a magnetizing station where the magnetizable ink is subjected to a magnetic field which magnetizes the ink in accordance with the unique geometry of the imprinted symbols. The symbols, having thus been magnetized, are each characterized by their own magnetic field which provides a basis for recognizing or reading them in a magnetic ink character recognition system.
2. Description of the Prior Art
Prior art magnetic ink character recognition systems such as disclosed in U.S. Pat. No. 3,103,646 issued to I. M. Sheafer et al, and U.S. Pat. No. 3,221,303 issued to R. W. Bradley, both of common ownership herewith, employ an electromagnetic transducer reading head having a single transverse gap which intercepts the magnetic fields of D.C. magnetized symbols of a standard font as they are moved in sequence past the reading head for producing a unique characteristic voltage waveform for each of the magnetized symbols. A particular symbol will provide greater or lesser amounts of magnetic flux in accordance with its particular shape as the quantity of flux is directly proportional to the ink area which has been magnetized. Therefore, each of the characters in the type font produces a distant voltage signal at the output of the transducer proportional to the time rate of change of flux according to their respective shapes. Such flux changes determine the polarity, spacing and amplitude of voltage signal peaks. This varying signal output from the transducer, after proper amplification and low pass filtering, is fed to a delay line which has eight taps or sample points which enable the symbol and its associated voltage waveform to be time divided into seven sections along the direction of document transport. Thus the leading edge is represented by the voltage signal at the first tap of the delay line while the trailing edge of the printed symbol is represented by the voltage signal at either the last tap, or some tap between the first and the last if the symbol is relatively narrow in width. Thus, when a character or symbol has been completely transported past a transducer head, a voltage value appears at each tap of the delay line which, if plotted graphically, would represent the characteristic voltage waveform for the symbol being read.
These voltage signals appearing at the taps of the delay line may be passed to a plurality of correlation networks, with each network associated with a unique symbol of the type font to be read. If a particular symbol is being read, then its associated correlation network will provide the highest voltage output relative to the other correlation networks of the system. This is accomplished by selectively choosing the taps of the delay line to be fed to the individual correlation networks and by providing different predetermined valued resistors to collectively represent the ideal waveform of the symbol associated with a particular correlation network.
In general, error in character recognition systems refers to undesirable correlation of a voltage waveform stored in the delay line with an ideal voltage waveform representing a character other than the character being read. This type of error may be conveniently referred to as a character misread. Misreads occur in magnetic ink character recognition systems that employ correlation techniques due to a combination of various factors. The prior art character recognition disclosed in the above-mentioned U.S. Pat. No. 3,221,303 issued to R. W. Bradley, employs an unexpected peak detector to effectively prevent misreading poorly imprinted symbols having extraneous magnetic material. However, it has been recently discovered that certain other imprinting faults result in character misreads that cannot be prevented by an unexpected peak detector alone.
A symbol imprinted on the document may be distorted such that portions of the symbol of character within the symbol outline are not covered with magnetic ink. Such a distortion may occur due to imperfections of the printing devices employed to imprint a character on a document. Also, the pigment of the magnetic ink used by the printing devices may not have been uniformly dispursed throughout the character outline. Such poorly defined or misprinted characters produce voltage waveforms that more nearly resemble the ideal waveform of a character other than the character that was attempted to be printed, thereby causing a misread.
Besides the possibility of poor definition of characters on a document due to faulty printing techniques, there are inherent limitations in the use of correlation techniques in character recognition systems. For example, there are unavoidable hardware component tolerances in the delay line utilized to store the representation of the voltage waveform that prevents the sampling of the stored information at the optimum time. Large variations of character line width when considered with the wide range of signal voltage levels that must be sensed further prohibit the optimum sampling of the voltage waveforms. Furthermore, there are insertion losses with frequency variations in the delay line that further degrade the performance of the character recognition system.
In spite of all the above listed limitations in a character recognition system employing correlation techniques, it is imperative that the system determines the occurrence of a poorly printed character being incorrectly read as some other character and provides an alarm or reject signal in response to which the document would be specially handled.